Split tags: Genrate maintainer-view tag too

Split tags: Genrate maintainer-view tag too

push_tagwants gets a new argument, $maintviewhead, which is defined
iff the quilt mode means we want split tags.  It then specifies the
generation of both tags.

push_mktags gains the ability to make the new `maint' view tag (and
checks that the view is not something else unexpected).

Introduce a function debiantag_maintview for calculating the
maintainer's idea of the tag.  Currently we use DEP-14 encoding (and
this function is a clone-and-hack of Dgit.pm::debiantag_old, but
perhaps we should do something more complicated (depending on the
quilt mode).

For now we do not push the maint view tag anywhere.  The ability to do
that, where appropriate, will come in a moment.

This commit introduces protocol version 4, which includes the new
`maint-view' param.  When have a split brain quilt mode, we need to be
sure that we're using protocol version 4 or out peer may not honour
this, resulting in an annoying failure later (the responder sending a
different number of tags to the number expected).

We also have to check that we're using the new tag format (or we might
want to try to generate two different tags with the same name, which
is madness).

When the quilt mode is not a split brain one, this whole commit should
produce no overall functional change (even though a higher protocol
version may be negotiated).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>